It's not a perfect, but it's enough for 99% use cases.
From Bugzilla:
fre.fbsdpr@expertmaker.com 2015-11-12 15:41:13 UTC
Hi,
When calling the script target with the ZFSBOOT_DISKS variable set, the zfsboot target will be run. This logic should also be used for the umount target, choosing 'zfs umount -a' to be run.